Unlike so many Android devices, on Galaxy S5 you have the option to specify whether to see the message preview on the lock screen on status bar or not. To disable message preview on lock screen / status bar on Galaxy S5 do as follows:
1. Open Messages application
2. On the main screen of the app, tap the 3dot icon and open Settings.
3. Open Notification settings
4. Under Preview Message settings, mark/unmark "Lock screen" and "Status bar" to enable/disable message preview